In the context of the present invention, the term niobium suboxide is to be understood as meaning compounds of the formula NbOz where z<2.2 and preferably 0.5<z<2.2.
Solid electrolyte capacitors with a very large active capacitor surface area and therefore a small overall size that is suitable for mobile communications electronics used are predominantly those with a niobium or tantalum pentoxide barrier layer applied to a corresponding conductive support, utilizing the stability of these compounds (“valve metal”), the relatively high dielectric constant and the fact that the insulating pentoxide layer can be produced electrochemically with a very uniform layer thickness. Metallic or conductive lower oxidic (suboxide) precursors of the corresponding pentoxides are used as carriers. The support, which simultaneously forms a capacitor electrode (anode), comprises a highly porous, sponge-like structure which is produced by sintering extremely fine particulate primary structures or secondary structures which are already in sponge-like form. The surface of the support structure is electrolytically oxidized (“formed”) to give the pentoxide, with the thickness of the pentoxide layer being determined by the maximum voltage used for the electrolytic oxidation (“forming voltage”). The counterelectrode is produced by impregnating the sponge-like structure with manganese nitrate, which is thermally converted into manganese dioxide, or with a liquid precursor of a polymer electrolyte and polymerization. The electrical contacts to the electrodes are formed on one side by a tantalum or niobium wire sintered in during production of the support structure and the metallic capacitor casing, which is insulated from the wire.
The capacitance C of a capacitor is calculated using the following formula:C=(F·ε)/(d·VF)where F is the capacitor surface area, ε is the dielectric constant, d is the thickness of the insulator layer per V of forming voltage and VF is the forming voltage. Since the dielectric constant ε is 27.6 for tantalum pentoxide and 41 for niobium pentoxide, but the growth in layer thickness per volt of forming voltage d is 16.6 and 25 Å/V respectively, both pentoxides have virtually the same quotient ε/d=1.64 and 1.69 respectively. Capacitors based on both pentoxides, with the same geometry of the anode structures, therefore have the same capacitance. Differences encountered in details of specific weight-related capacitances are trivial, resulting from the different densities of Nb, NbOx and Ta. Therefore, anode structures made from Nb and NbOx have the advantage of saving weight when used, for example, in mobile telephones, which strive for every single gram of weight saved. For cost reasons, NbOx (Niobium suboxide) is more favourable than Nb, since part of the volume of the anode structure is formed by oxygen.
One drawback of niobium suboxide as support body for capacitor barrier layers is that a sufficient compressive strength of the sintered anode body and a sufficient wire tensile strength are only achieved by sintering the pressed bodies at a relatively high sintering temperature (in the region of 1450° C. compared to 1150° C. in the case of Nb metal). The high sintering temperature leads firstly, as a result of increased surface diffusion, to a decrease in the surface area of the pressed body during transition to the sintered body, and therefore to a lower capacitance, and secondly requires increased levels of energy and increased loading being applied to the materials of the crucibles and sintering furnaces.
The reason is that niobium suboxide, by comparison with niobium metal with metallic ductility, already has considerable covalent bond levels, which produce in relative terms a ceramic brittleness.
Furthermore, the compressive strength of the anode bodies prior to sintering leaves something to be desired, since the porous powder agglomerates do not stably “mesh together” during pressing, but rather have an increased tendency to disintegrate or abrade, with the result that not only is the formation of stable sintered bridges impeded, but also agglomerates in a more finely particulate form, even down to isolated primary particles, are formed, causing an adverse change in the pore structure of the sintered anode body. Furthermore, there is increased wear to the press tools be comparison with metal powders. By no means least, niobium oxide powders also have worse flow properties than metal powders, making it more difficult to meter the powders into the press tools.
According to WO 01/71738 A2, therefore, it is attempted to relieve the magnitude of these drawbacks by on the one hand adding lubricants and binders during pressing of the powders, which are intended to compensate for the drawback of insufficient compressive strength of the pressed bodies, and on the other hand by using more finely particulate agglomerates of primary particles, which are less likely to fracture, but this is to the detriment of the pore structure.